The present invention relates to the oxidation of p-methyl-substituted hindered phenols. More particularly, the present invention relates to a process for the heterogeneous oxidation of p-methyl-substituted phenols that is flexible in operation allowing the selective preparation of various oxygenated organic products including nucleophilic addition products and coupled reaction products. The latter class of coupled reaction products includes substituted stilbenequinones and reduced reaction products thereof.
Processes for the preparation of substituted stilbenequinones from phenolic compounds employing chemical oxidizing agents have been previously described. V. Balogh et al., J. Org. Chem., 36, 1339 (1971) described such a process employing as an oxidizing agent silver carbonate on celite. The substituted stilbenequinones prepared by the process were reduced to the corresponding 4,4'-dihydroxystilbenes by use of zinc and acetic acid.
The addition of nucleophiles to intermediate species formed by oxidation of phenols is previously known. C. D. Cook et al., J.A.C.S., 78, 3797 (1956) reported the preparation of benzyl radicals by oxidation of substituted p-methylphenols with alkaline ferricyanide or lead dioxide. Dimeric products as well as electrophilic addition products from the addition of nucleophiles, e.g., alcohols, were also reported.
Similarly, L. J. Filar, Tet. Lett., (25), 9-16 (1960) reported the preparation of quinone methides from p-methyl phenols by action of a base such as triethylamine or by quantitative amounts of silver oxide or lead oxide. The mechanism was assumed to require the formation of an aryloxy radical. Reaction products in the absence of nucleophiles were dimeric materials. In the presence of nucleophilic reactants such as alcohols or carboxylic acids, addition products were formed.
Braxton et al., U.S. Pat. No. 3,213,114 (1965) disclose that the oxidation of 2,6-di-tert-butyl-4-methylphenol is problematical, leading to the formation of an extensive variety of products. Thus, in their invention directed to the preparation of p-benzoquinones, they teach that it is necessary that each of the 2,4 and 6 substituents on 2,4,6-trihydrocarbyl-substituted phenols be tertiary (i.e., having no hydrogen atoms attached to the carbon directly bonded to the phenol ring).
Bauer et al., U.S. Pat. No. 4,119,671 (1978) teach the preparation of salicylaldehyde and other hydroxybenzaldehydes by oxidation of a hydroxybenzyl alcohol with a platinum and lead-, silver-, tellurium- or tin-activated catalyst in an aqueous alkaline solution.
Nishizawa et al., U.S. Pat. No. 4,429,163 (1984) teach the preparation of certain 4-hydroxybenzaldehydes from oxidizing corresponding 4-methylphenols with a cobalt catalyst in solvents stable to oxygen and capable of dissolving the starting material, preferably such as alcohols, hydrocarbons, ethers, halogenated hydrocarbons and amines.
Each of the foregoing references is hereby incorporated by reference.
What is lacking and what is needed is a process for the selective heterogeneous catalytic oxidation of organic 4-methyl-substituted hindered phenols. By hindered is meant that there are any of a broad variety of substituents on at least the 2 and 6 positions of the phenol ring.